1. Field of the Invention
The present invention relates to RF power amplifiers.
2. State of the Art
Battery life is a significant concern in wireless communications devices such as cellular telephones, pagers, wireless modems, etc. Radio-frequency transmission, especially, consumes considerable power. A contributing factor to such power consumption is inefficient power amplifier operation. A typical RF power amplifier for wireless communications operates with only about 10% efficiency. Clearly, a low-cost technique for significantly boosting amplifier efficiency would satisfy an acute need.
A power amplifier typically includes multiples stages, for example a final output stage and one or more pre-amplifier or driver stages. Much work has been done on maximizing the efficiency of the final output stage. A significant improvement in the efficiency of the final output stage was achieved with the advent of the Class E power amplifier, described in U.S. Pat. No. 3,919,656, incorporated herein by reference. In a Class E amplifier, the current and voltage waveforms of a switch are phased such that during switching one of the two quantities is at or near zero, minimizing power dissipation.
The Class E amplifier established the operation and design of the final stage of an amplifier operating in switch-mode. As a result, it is well-understood in the art of RF power amplifiers that to improve the conversion efficiency of supplied DC power to output power, the amplifier must be operated in a nonlinear manner—the most nonlinear operation possible of an amplifying element (such as a transistor) being operation as a switch. Indeed, the reported output efficiencies of switch-mode RF power amplifiers (e.g., Class E) are significantly higher (e.g., 80%) than mildly nonlinear amplifiers such as Class AB (e.g., 45%).
To operate an RF power amplifier in switch mode, it is necessary to drive the output transistor(s) rapidly between cutoff and full-on, and then back to cutoff, in a repetitive manner. The means required to achieve this fast switching is dependent on the type of transistor chosen to be used as the switch: for a field-effect transistor (FET), the controlling parameter is the gate-source voltage, and for a bipolar transistor (BJT, HBT) the controlling parameter is the base-emitter current.
Various designs have attempted to improve on different aspects of the basic Class E amplifier. One such design is described in Choi et al., A Physically Based Analytic Model of FET Class-E Power Amplifiers—Designing for Maximum PAE, IEEE Transactions on Microwave Theory and Techniques, Vol. 47, No. 9, September 1999, incorporated herein by reference. This contribution models various non-idealities of the FET switch and from such a model derives conclusions about advantageous Class E amplifier design. For the chosen topology, maximum power-added efficiency (PAE) of about 55% occurs at a power level of one-half watt or less. At higher powers, PAE is dramatically reduced, e.g., less than 30% at 2 W.
The PAE of a power amplifier is set by the amount of DC supply power required to realize the last 26 dB of gain required to achieve the final output power. (At this level of gain, the power input to the amplifier through the driving signal—which is not readily susceptible to measurement—becomes negligible.) Presently, there are no known amplifying devices capable of producing output powers of 1 W or greater at radio frequencies and that also provide a power gain of at least 26 dB. Accordingly, one or more amplifiers must be provided ahead of the final stage, and the DC power consumed by such amplifiers must be included in the determination of overall PAE.
Conventional design practice calls for an amplifier designer to impedance-match the driver output impedance to the input impedance of the final switching transistor. The actual output power therefore required from the driver stage is defined by the required voltage (or current) operating into the (usually low) effective input impedance of the switching element. A specific impedance for the input of the switching transistor is not definable, since the concept of impedance requires linear operation, and a switch is very nonlinear.
An example of an RF amplifier circuit in accordance with the foregoing approach is shown in FIG. 1. An interstage “L section” consisting of an inductor L1, a shunt capacitor C and an inductor L2 is used to match the driver stage to an assumed 50 ohm load (i.e., the final stage).
This conventional practice treats the interstage between the drive and final stages as a linear network, which it is not. Further, the conventional practice maximizes power transfer between the driver and final stages (an intended consequence of impedance matching). Thus, for example, in order to develop the required drive voltage for a FET as the switching transistor, the driver must also develop in-phase current as well to provide the impedance-matched power.
Another example of a conventional RF power amplifier circuit is shown in FIG. 2. This circuit uses “resonant interstage matching” in which the drive and final stages are coupled using a coupling capacitor Ccpl.
As noted, conventional design practice fails to achieve high PAE at high output power (e.g., 2 W, a power level commonly encountered during the operation of a cellular telephone). A need therefore exists for an RF power amplifier that exhibits high PAE at relatively high output powers.